This invention relates to a commercially advantageous process in accordance with which a precursor of phenylalanine can be prepared with a high yield.
L-Phenylalanine is a nutrient, one of essential amino acids, and often used in for infusion solutions. Also, it has an important use as an intermediate for synthesis of L-aspartyl phenylalanine methylester, which is one of the artificial sweeteners.
It is well-known in the art that L-phenylalanine is prepared by an optical or racemic resolution of DL-phenylalanine (hereinunder referred to as phenylalanine), which is in turn prepared by way of a synthetic method using phenylacetaldehyde as a starting material. The phenylacetaldehyde, however, is expensive and, as it is, thermally or chemically unstable, so that the handling in a commercial scale is difficult.
On the other hand, bisulfite addition salts of phenylacetaldehyde may be used as the starting material for synthesis of phenylalanine and the pure compound thereof is commercially easy-to-handle powders.
The phenylacetaldehyde has, hitherto, been produced industrially by an isomerization reaction of styrene oxide, while lower-cost processes of the production are progressively developed using direct oxidation of styrene, hydrolysis of .beta.,.beta.-dihaloethylbenzene, a hydroformylation reaction of benzyl halide and the like.
In these processes, the most effective way of recovering pure phenylacetaldehyde from the reaction system may be carried out by a contact treatment of the reaction solution with bisulfite or its solution followed by separation and recovery of phenyl acetaldehyde as the bisulfite addition salt. Therefore, such a bisulfite addition salt of phenylacetaldehyde is considered as an important starting material for synthesis of phenylalanine.
It is also known in the art that 5-benzylhydantoin, precursor of phenylalanine, is synthesized from a bisulfite addition salt of phenylacetaldehyde (Gaudry; Canadian Journal of Research, vol. 26(B) p.p 773.about.776 (1948)). In this synthesis, 5-benzylhydantoin with low yield, that is 42% or so, is obtained by reacting in a 50% aqueous solution of ethanol a 1:2:4 mixture of a bisulfite addition salt of phenylacetaldehyde, potassium cyanide and ammonium carbonate.